diff --git a/backend/src/entities/user-secret/repository/user-secret-repository.extension.ts b/backend/src/entities/user-secret/repository/user-secret-repository.extension.ts
index e381da272..61e37c44a 100644
--- a/backend/src/entities/user-secret/repository/user-secret-repository.extension.ts
+++ b/backend/src/entities/user-secret/repository/user-secret-repository.extension.ts
@@ -1,8 +1,28 @@
-import { FindOptionsWhere, Like } from 'typeorm';
+import { DeepPartial, FindOptionsWhere, Like } from 'typeorm';
 import { UserSecretEntity } from '../user-secret.entity.js';
 import { IUserSecretRepository } from './user-secret-repository.interface.js';
 
 export const userSecretRepositoryExtension: IUserSecretRepository = {
+	/**
+	 * Atomically provisions a secret the first time it is needed and returns whichever row exists
+	 * afterwards — the one just inserted or the one a concurrent caller inserted first.
+	 * `user_secrets (companyId, slug)` is UNIQUE; a read-then-insert races with itself when several
+	 * requests provision the same slug at once (e.g. the universal-backend instances all fetching a
+	 * fresh site's end-user signing key on its first page load) and the losers fail with
+	 * `duplicate key value violates unique constraint "IDX_f39a47aac503fe096b0c77f2b3"`.
+	 * `INSERT … ON CONFLICT DO NOTHING` + re-read makes every caller land on the single row.
+	 */
+	async createSecretUnlessExists(
+		values: DeepPartial<UserSecretEntity> & { slug: string; companyId: string },
+	): Promise<UserSecretEntity> {
+		await this.createQueryBuilder().insert().into(UserSecretEntity).values(values).orIgnore().execute();
+		const secret = await this.findOne({ where: { slug: values.slug, companyId: values.companyId } });
+		if (!secret) {
+			throw new Error(`Secret "${values.slug}" is missing right after its creation.`);
+		}
+		return secret;
+	},
+
 	async findSecretBySlugAndCompanyId(slug: string, companyId: string): Promise<UserSecretEntity | null> {
 		return this.findOne({
 			where: {
diff --git a/backend/src/entities/user-secret/repository/user-secret-repository.interface.ts b/backend/src/entities/user-secret/repository/user-secret-repository.interface.ts
index 865c2b099..0a2b92e0f 100644
--- a/backend/src/entities/user-secret/repository/user-secret-repository.interface.ts
+++ b/backend/src/entities/user-secret/repository/user-secret-repository.interface.ts
@@ -1,7 +1,11 @@
-import { Repository } from 'typeorm';
+import { DeepPartial, Repository } from 'typeorm';
 import { UserSecretEntity } from '../user-secret.entity.js';
 
 export interface IUserSecretRepository {
+	createSecretUnlessExists(
+		this: Repository<UserSecretEntity>,
+		values: DeepPartial<UserSecretEntity> & { slug: string; companyId: string },
+	): Promise<UserSecretEntity>;
 	findSecretBySlugAndCompanyId(
 		this: Repository<UserSecretEntity>,
 		slug: string,
diff --git a/backend/src/microservices/sitenova-microservice/services/sitenova-enduser-auth.service.ts b/backend/src/microservices/sitenova-microservice/services/sitenova-enduser-auth.service.ts
index d455f7f55..b6889410f 100644
--- a/backend/src/microservices/sitenova-microservice/services/sitenova-enduser-auth.service.ts
+++ b/backend/src/microservices/sitenova-microservice/services/sitenova-enduser-auth.service.ts
@@ -118,16 +118,18 @@ export class SitenovaEndUserAuthService {
 		if (existing) {
 			return Encryptor.decryptData(existing.encryptedValue);
 		}
-		const key = crypto.randomBytes(32).toString('hex');
-		const secret = this._dbContext.userSecretRepository.create({
+		// First use of this connection's key. Concurrent first uses (the universal-backend replicas
+		// all fetching a fresh site's credentials on its first page load) all miss the read above, so
+		// the insert is conflict-tolerant and the key handed back is whichever row won — never a
+		// locally generated key that the database did not keep.
+		const secret = await this._dbContext.userSecretRepository.createSecretUnlessExists({
 			slug,
-			encryptedValue: Encryptor.encryptData(key),
+			encryptedValue: Encryptor.encryptData(crypto.randomBytes(32).toString('hex')),
 			companyId,
 			expiresAt: null,
 			masterEncryption: false,
 			masterHash: null,
 		});
-		await this._dbContext.userSecretRepository.save(secret);
-		return key;
+		return Encryptor.decryptData(secret.encryptedValue);
 	}
 }
diff --git a/backend/test/ava-tests/non-saas-tests/non-saas-sitenova-microservice-e2e.test.ts b/backend/test/ava-tests/non-saas-tests/non-saas-sitenova-microservice-e2e.test.ts
index ee38b8914..ae6295b5c 100644
--- a/backend/test/ava-tests/non-saas-tests/non-saas-sitenova-microservice-e2e.test.ts
+++ b/backend/test/ava-tests/non-saas-tests/non-saas-sitenova-microservice-e2e.test.ts
@@ -9,8 +9,11 @@ import cookieParser from 'cookie-parser';
 import jwt from 'jsonwebtoken';
 import nock from 'nock';
 import request from 'supertest';
+import { DataSource } from 'typeorm';
 import { ApplicationModule } from '../../../src/app.module.js';
+import { BaseType } from '../../../src/common/data-injection.tokens.js';
 import { WinstonLogger } from '../../../src/entities/logging/winston-logger.js';
+import { UserSecretEntity } from '../../../src/entities/user-secret/user-secret.entity.js';
 import { TableActionEventEnum } from '../../../src/enums/table-action-event-enum.js';
 import { TableActionMethodEnum } from '../../../src/enums/table-action-method-enum.js';
 import { TableActionTypeEnum } from '../../../src/enums/table-action-type.enum.js';
@@ -536,3 +539,64 @@ test.serial(`${currentTest} rejects a missing microservice JWT (401) and a malfo
 		.set('Accept', 'application/json');
 	t.is(noKeys.status, 400);
 });
+
+currentTest = 'GET /internal/sitenova/connection/:connectionId (internal, microservice JWT)';
+
+// The universal-backend asks for a site's credentials + end-user signing key on every cold cache,
+// and a generated site's first page load fires several such requests at once (one per replica,
+// one per parallel API call). The key is provisioned lazily on the first request, so the first
+// burst must be race-safe: every caller gets 200 and the SAME key, and exactly one
+// `sitenova:enduser-jwt:<connectionId>` secret exists afterwards — not the
+// `duplicate key value violates unique constraint "IDX_f39a47aac503fe096b0c77f2b3"` 500 the
+// read-then-insert used to produce for every loser of the race.
+test.serial(`${currentTest} concurrent first requests provision one signing key`, async (t) => {
+	const { connectionId, userId } = await createConnectionAndTable();
+
+	const responses = await Promise.all(
+		Array.from({ length: 8 }, () =>
+			request(app.getHttpServer())
+				.get(`/internal/sitenova/connection/${connectionId}`)
+				.set('Authorization', microserviceAuthHeader())
+				.set('Accept', 'application/json'),
+		),
+	);
+
+	for (const res of responses) {
+		t.is(res.status, 200, res.text);
+		t.is(res.body.connection.id, connectionId);
+		t.is(typeof res.body.endUserJwtKey, 'string');
+	}
+	const keys = new Set(responses.map((res) => res.body.endUserJwtKey as string));
+	t.is(keys.size, 1, 'every concurrent request resolved to the single stored signing key');
+	const [key] = keys;
+
+	const dataSource = app.get<DataSource>(BaseType.DATA_SOURCE);
+	const storedSecrets = await dataSource
+		.getRepository(UserSecretEntity)
+		.count({ where: { slug: `sitenova:enduser-jwt:${connectionId}` } });
+	t.is(storedSecrets, 1, 'the signing key is stored once');
+
+	// The key handed out is the one the core itself signs with: a visitor token minted by the
+	// core's register route verifies with it, so tokens issued by either service stay interchangeable.
+	const usersTable = await createUsersTable(connectionId, userId);
+	const visitorToken = await registerAndGetToken(connectionId, usersTable);
+	const decoded = jwt.verify(visitorToken, key, { algorithms: ['HS256'], audience: 'sitenova:enduser' }) as {
+		cid: string;
+	};
+	t.is(decoded.cid, connectionId);
+
+	const again = await request(app.getHttpServer())
+		.get(`/internal/sitenova/connection/${connectionId}`)
+		.set('Authorization', microserviceAuthHeader())
+		.set('Accept', 'application/json');
+	t.is(again.status, 200);
+	t.is(again.body.endUserJwtKey, key, 'a later request returns the stored key, not a fresh one');
+});
+
+test.serial(`${currentTest} is rejected (401) without a microservice JWT`, async (t) => {
+	const { connectionId } = await createConnectionAndTable();
+	const res = await request(app.getHttpServer())
+		.get(`/internal/sitenova/connection/${connectionId}`)
+		.set('Accept', 'application/json');
+	t.is(res.status, 401);
+});